I need to pass a structure to a pthread and be able to change the values of the struct from the function the pthread will execute. This is my code:

#include <stdio.h>
#include <stdlib.h>
#include <vector>
#include <pthread.h>

void *deal_cards(void* deck);

int main() {
    struct t_data {
        std::string name;
        std::string status;
        std::vector<int> hand;
        std::vector<int> *ptr_deck;
    };
    std::vector<int> deck = {1,2,3};
    std::vector<int> *p_deck = &deck;
    struct t_data player1_data = {"PLAYER 1", "lose", {}, p_deck};
    struct t_data *player1 = &player1_data;
    pthread_t p1;
    pthread_create (&p1, NULL, deal_cards, (void *) player1);
}

void* deal_cards (void* data) {
    (struct t_data*)->(std::vector<int>*)ptr_deck.push_back(3);
}

I get the following error when I run this

In function 'void* deal_cards (void*)
error: expected primary-expression before 'struct'
error: expected ')' before 'struct'

In case it matters I'm compiling on Linux with

g   main.cpp -o main -lpthreads

What am I missing and is that the right way to alter the values inside the structure?

CodePudding user response：

There are numerous mistakes in your code:

• The t_data structure type is defined local to main(), so deal_cards() can't use it.

• main() is exiting, destroying its local variables, while the thread is still running.

• the syntax you are trying to use to access push_back() in deal_cards() is all wrong. You are not referencing the data input parameter at all, that is what you should be type-casting to t_data*. And, you are type-casting ptr_deck to std::vector<int>*, which it is already typed as, so that cast is unnecessary. And, since ptr_deck is a pointer, you need to use the -> operator to access its push_back() method, not the . operator.

Also, while not strictly errors, you should also be aware of these:

• you are using std::string without #include <string>

• unlike in C, in C you don't need to prefix references to a structure type with the struct keyword. Only the declaration of the structure type needs to use the struct keyword.

With that said, try this instead:

#include <vector>
#include <string>
#include <pthread.h>

void* deal_cards(void* deck);

struct t_data {
    std::string name;
    std::string status;
    std::vector<int> hand;
    std::vector<int> *ptr_deck;
};

int main() {
    std::vector<int> deck = {1,2,3};
    t_data player1_data = {"PLAYER 1", "lose", {}, &deck};
    pthread_t p1;
    if (pthread_create (&p1, NULL, deal_cards, &player1_data) == 0) {
        pthread_join (p1, NULL);
        // use deck as needed...
    }
}

void* deal_cards (void* data) {
    static_cast<t_data*>(data)->ptr_deck->push_back(3);
    return NULL;
}

Though, you really should be using C 's own std::thread class instead of pthreads directly:

#include <vector>
#include <string>
#include <thread>

struct t_data {
    std::string name;
    std::string status;
    std::vector<int> hand;
    std::vector<int> *ptr_deck;
};

void deal_cards(t_data* deck);

int main() {
    std::vector<int> deck = {1,2,3};
    t_data player1_data = {"PLAYER 1", "lose", {}, &deck};
    std::thread p1(deal_cards, &player1_data);
    p1.join();
    // use deck as needed...
}

void deal_cards (t_data* data) {
    data->ptr_deck->push_back(3);
}